The notion of improving paper absorbent structures by incorporating therein a highly absorbent material is described in the art.
Amongst the disclosures which may be found, U.S. Pat. No. 4,252,761, Schoggen et al, issued Feb. 24, 1981, discloses that sheets may be prepared from certain modified fibrous carboxymethylcellulose derivatives, sometimes known as bibulous cellulosic fibers. Such sheets are described in patents including U.S. Pat. No. 3,678,031, Schoggen, issued July 18, 1972 and U.S. Pat. No. 3,589,364, Dean and Ferguson, issued June 29, 1971. Mixtures of these modified fibers unmodified fibers are also disclosed in '761. The process of preparing the sheets comprises the steps of air-laying the fibers to form an airfelt, increasing the moisture content of the airfelt, and compacting the moisturized airfelt between a pair of opposed pressure-loaded rollers or between two planar members such as hydraulic rams.
Another approach, as taught in U.S. Pat. No. 4,295,987, Parks, issued Oct. 20, 1981, involves making a two-ply paper towel containing an air-laid, bonded, powdered particulate absorbent copolymer, the latter for example comprising acrylic acid, a crosslinking monomer, and a crosslinking agent. A layer of the particulate absorbent copolymer is sandwiched between two paper plies, each preformed according to the wet-laying process taught in U.S. Pat. No. 3,301,746, Sanford et al, issued Jan. 31, 1967. See '987, Example 2.
U.S. Pat. No. 4,076,663, Masuda et al, issued Feb. 28, 1978, disclose yet another highly water-absorbent material, having resinous form and prepared by polymerizing three components: a component chemically identified as starch or cellulose; a monomer having a polymerizable double bond which is water-soluble or becomes water-soluble by hydrolysis; and a crosslinking agent. The resulting product is then hydrolyzed if necessary. See Example 5, wherein the first component is "fluff pulp", the second component is a mixture of acrylic acid and sodium acrylate and the third component is N,N-methylenebisacrylamide. In this example, a ceric ammonium nitrate polymerization catalyst is also used. It is broadly stated that such resinous materials may be applied to absorbent articles, such as paper towels, by various methods including mixing the powdered resin with materials such as pulp, spraying an aqueous dispersion of the resin onto substrates such as pulp or paper, or immersing such substrates in aqueous dispersions of the resin followed by kneading and drying.
Brandt et al, U.S. Pat. No. 4,654,039, issued Mar. 31, 1987, reissued as U.S. Pat. No. Re 32,649 on Apr. 19, 1988, refer to a number of disclosures in the art of "hydrogel-forming materials which essentially comprise only cross-linked polymerized unsaturated monomers," in the sense that these materials have no starch or cellulose moieties. It is disclosed that water-extractable polymer material in such materials is undesirable. Improved "hydrogel-forming" materials, also of the non-cellulosic variety, having high gel volume, high gel strength, and low levels of extractable polymer, are the subject of the Brandt et al invention. These improved materials include a substantially water-insoluble, slightly cross-linked, partially neutralized, hydrogel-forming polyacrylate polymer. It is disclosed that various gross morphologies of the polymer, including fibers, can be used in an air-laid or laminated absorbent structure suitable for use in an absorbent article, the latter being defined as including, inter alia, diapers, paper towels and facial tissues.
Weisman, U.S. Pat. No. 4,610,678, issued Sept. 9, 1986, discloses flexible, substantially unbonded absorbent structures, especially useful for making thin diaper cores. These structures comprise what is described as a mixture of hydrophilic fibers and discrete particles of a water-insoluble hydrogel. Examples of the former include cellulose fibers and hydrophilized thermoplastic fibers; the latter are generally chemical compounds, including hydrolyzed acrylonitrile grafted starch as well as polyacrylates, which it is stated can be in fibrous form. The absorbent structures are generally prepared by dry processing, such as air-laying followed by compression. Weisman characterizes the notion of wet-laying mixtures of hydrogel particles and hydrophilic fibers as disadvantageous, in comparison with his invention. The specific disadvantages of wet-laying the hydrogel apparently include stiffness of the wet-laid product because of undesirable fiber-fiber bonding, as well as expense incurred by added drying load. Poor wicking characteristics of hydrogels and restricted swelling of hydrogel particles in a fibrous matrix are referred to as two possible causes for poorly performing hydrogel-containing webs.
Saotome, EP-A 192,216, published Aug. 27, 1986, describes another water-absorbent fibrous structure, characterized as comprising a fibrous cellulosic material impregnated with a waterabsorbent acrylic polymer and a fibrous material, which is produced by a method in which an aqueous solution of a monomeric component comprising acrylic acid and a radical initiator is diffused in a fibrous cellulosic material and heated, followed by blending with a fibrous material. It is stated that this material finds application in various absorbent articles, including disposable diapers and paper towels. There is not, apparently, any criticality in the kind of fibrous cellulosic material or fibrous material. The former may, for example, be a chemically purified wood pulp; the latter may be identical with the former or may comprise a synthetic fiber such as polyester. See Example 7, wherein acrylic acid, sodium hydroxide, N,N-methylenebisacrylamide and potassium persulfate are mixed and reacted in water, the resulting solution is sprayed over a chemically purified wood pulp, heated to obtain a polymerization product, and dried. This intermediate product is mixed with wood pulp, water and methanol and "vigorously stirred to bring the polymerization product to pieces." The mixture is wet-laid on a wire netting to make a sheet, and the sheet is dried.
Nakanishi et al, U.S. Pat. No. 4,721,647, issued Jan. 26, 1988, discloses absorbent articles comprising a base material of fibers, a part or all of which fibers are hydrophobic; and a water-absorbent polymer, a part or all of which is in the form of substantially spherical particles bonded to said fibers to surround them. Though little preparative detail is given, the article is apparently made by steps comprising spraying droplets of monomer onto the fibers, polymerizing such as by heat-curing, and drying.
In addition to the foregoing, there have been many other attempts to modify the properties of paper, both before the papermaking operation (such as by grafting fibers followed by wet-laying to form modified paper), as well as after the paper-making operation (such as by wet-laying fiber furnishes followed by grafting the paper). See, for example, "Chemical Modification of Papermaking Fibers," K. Ward, Jr., Marcel Dekker, N.Y., 1973. At page 171, Ward states: "Actual industrial utilization of grafting for paper has been minimal, although much time, money and effort have been spent in investigating the subject." At page 183, Ward states:". . . grafting pulp with polymers which have a high affinity for water does not harm -and may even improve- the mechanical properties of paper made therefrom."
See also the more-recent disclosures of "The Chemistry and Technology of Cellulosic Copolymers," A. Hebeish and J. T. Guthrie, Springer-Verlag, New York, 1981. At page 336 it is stated: "There has been no evidence of the successful utilization of true grafting in the paper industry proper, although the Lenzing development of a fiber for nonwoven webs comes close to it." (In connection with the Lenzing material, see p335, paragraph 3 and cited).
See also U.S. Pat. No. 4,354,901, Kopolow, issued Oct. 19, 1982 and U.S. Pat. No. 4,552,618, Kopolow, issued Nov. 12, 1985. The Kopolow disclosures relate to compression or heat treatment of boards in the dry state after a wet-laying papermaking process. The boards comprise "hydrocolloidal fibers" such as those of U.S. Pat. No. 3,889,678, Chatterjee et al, issued June 17, 1975.
It is an object of the present invention to provide wet-laid absorbent paper structures in the form of paper webs which are capable of quickly absorbing and tenaciously retaining appreciable quantities of water and other aqueous fluids, yet are soft to the touch when dry and pleasant to handle even when wet, especially in not shedding particles of absorbent material or feeling unpleasantly "gel-like" when in use.
It is a further object to provide disposable absorbent articles having similar advantages, such articles including layered or homogeneous single-ply tissues, towels and wipes as directly made by the present process, as well as the products of conventionally converting the paper webs of the invention, e.g., by combining into a multi-ply article one or more plies of the absorbent paper structures of the invention. Such converted disposable absorbent articles include multi-ply tissues and towels or wipes.
In another mode, it is an object herein to apply the invention for the more economical provision of absorbent articles than was hitherto possible without sacrificing their water-absorbency, aesthetics or usefulness.
It is a further object of the invention to provide an improved continuous wet-laying papermaking process for producing the absorbent paper structures economically.
It is yet another object of the invention to provide processes for making particularly preferred polymer-modified fibrous pulps, as well as the products thereof such as a particularly made hydrolyzed, methyl acrylate grafted kraft pulp, being especially desirable as a fibrous pulp for use in admixture with conventional pulps in the absorbent paper structures of the invention.